The present invention relates to an air flow meter and, more particularly, to a thermal type air flow meter for internal combustion engines which constitutes a part of an air intake system, and which is suitable to detect an air flow rate in the air intake system and control a fuel injection amount corresponding to the air flow rate.
An example of a conventional thermal type air flow meter for internal combustion engines is disclosed in JP-A-1-206223, wherein the air flow meter comprises a main air flow passage, a bypass air flow passage in the main air flow passage having air flow sensing elements disposed therein. The having bypass air flow passage consists of an axial bypass air flow passage in which air flows in the axial direction and in which the air flow sensing elements are disposed. A radial bypass air flow passage intersects the axial bypass air flow passage at right angles, whereby an inverted T-letter bypass air flow passage is formed in the main air flow passage.
In the conventional air flow meter, complicated eddies occur around the intersection of the axial bypass air flow passage and the radial bypass air flow passage by impinging the air flow from the axial bypass air flow passage on a bottom wall of the radial bypass air flow passage and by abruptly changing the air flow direction at intersecting surfaces of the two bypass air flow passages. The complicated eddies cause unsteady change in pressure, which causes fluctuation in flow rate where the sensing elements are disposed. The sensing elements sense the fluctuation in flow rate, thereby increasing output noise of the sensor.